For connecting a DC power supply such as a photovoltaic panel or a storage battery and an AC electric path, a power conversion device which performs DC/AC power conversion through high-frequency switching of semiconductor switches is used. The power conversion device has a breaking device for overcurrent protection, on an output electric path (AC side), for example. As the breaking device, a circuit breaker or a fuse can be used. It is preferable to use a fuse from the perspective of cost performance because it is very rare for the breaking device to actually interrupt overcurrent and a fuse is lower in cost than a circuit breaker.
There are also high-function fuses capable of outputting a contact signal at the time of melting. Also here, from the perspective of cost performance, a simple fuse not having such a function is preferable. In the case of using such a fuse, melting of the fuse can be detected through situation determination using, for example, a CPU (Central Processing Unit) based on voltages on both sides of the fuse (see, for example, Patent Literature 1). The situation determination refers to performing comparison with a threshold value and determination as to whether or not there is periodic change, with respect to each of the primary-side voltage and the secondary-side voltage of the fuse.